add description, example, and install info

README.md

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+# Windmolen
+
+Treat multiple asynchronous iterators like they are a single asynchronous iterator.
+
+**Example:**
+
+```python
+import asyncio
+from windmolen import FanIn
+
+
+async def async_iterator_1():
+    for i in range(1, 5):
+        yield i
+        await asyncio.sleep(0.5)
+
+
+async def async_iterator_2():
+    for j in range(5, 9):
+        yield j
+        await asyncio.sleep(1)
+
+
+async def main():
+    async with FanIn(async_iterator_1(), async_iterator_2()) as async_iterators:
+        async for item in async_iterators:
+            print(item)
+
+
+if __name__ == "__main__":
+    asyncio.run(main())
+```
+
+## Install
+
+```shell
+pip install windmolen
+```